The present invention relates to the art of induction heating and, more particularly to the heat treating of valve seat inserts of an engine component such as an engine head.
The invention is suitable for valve seat inserts which are heat treated after in-head machining of the valve seat surface, and particularly if the valve seat insert is subject to loosening under operating conditions and will be described with reference thereto; however, it will be appreciated that the invention has broader applications and may be used for the heat treating of various hardenable inserts in both ferrous and non-ferrous components wherein secure mechanical retention of the insert is required under operating conditions.
It has become commonplace to utilize hardened valve seat inserts in internal combustion engine heads which coact with the reciprocating intake and exhaust valves to control the flow of intake and exhaust gases to and from the engine combustion chamber. This is particularly true where the temperature and operation cause accelerated wear of the valve seat surface if formed directly on the engine head. Therein, the strength and wear properties of the seating surface are not sufficient to provide acceptable service life. This is manifest in aluminum components but also arises in case iron and other ferrous components operating under severe service conditions. To overcome these deficiencies, roughtly machined valve seat inserts which are inserted into the engine head, accurately machined to final size and subsequently heat treated have become the preferred assemblies. The method and apparatus as disclosed in U.S. Pat. No. 4,438,310, assigned to the assignee of the present invention and is incorporated hereby by reference, is exemplary of a successful approach for accurately and uniformly providing accurately hardened valve seat surfaces yielding extended service life.
In such assemblies, the partially machined valve seat insert is compressively retained in a counterbore in the engine. The compressive retention force is provided by diametral interference between the outer diameter of the valve seat insert and the inner diameter of the counterbore and is generally in the range of 0.003 to 0.007 inches. This compressive retention may be provided by fitting the insert into the counterbore or alternatively by cryogenically cooling the insert to establish a diametral clearance between the insert and the bore, inserting the cooled insert into the bore, and allowing the insert to warm to ambient conditions. The valve seat insert is thereafter machined to size. During the subsequent inductive heating, the insert-head interface temperature is controlled by conduction and/or magnetic shielding or control of the power level to prevent excessive heating which would damage the engine head and destroy the pressure fit with the insert. These have proven to be effective techniques, particularly for aluminum heads.
Nonetheless, it has been found that some of the inserts are nonetheless subject to loosening in service, notwithstanding apparently satisfactory post assembly inspection. Inasmuch as loosening in service can cause substantial engine damage and is costly and time consuming to repair, there is a need to further improve the service retention of the inserts.
One of the largest contributors to valve seat loosening is the loss of compressive retention force during the inductive heating cycle. This can occur when the thermal expansion of the insert deforms and enlarges the bore diameter. Therein, the valve seat insert expands during the inductive heating whereas the engine head is not similarly affected to a similar extent due to the supplemental cooling and/or heat sink mass of the head, notwithstanding a greater thermal coefficient of expansion as is the case for an aluminum head. As a result of ambient temperatures, a loss of interference will be realized. Moreover, during engine operation, both the insert and the head reach a greater temperature equilibrium. Thereat, the differences in the expansion coefficients result in the bore diameter expanding at a greater rate than the insert and a further reduction or complete loss of retention. Under dynamic operation, the insert can loosen or eject resulting in engine failure.